Add an example for scatter plots with auto legends

[seisman]

Description of proposed changes

Preview: https://pygmt-git-gallery-scatter.gmt.vercel.app/gallery/plot/scatter.html

Add a new example, modified from https://matplotlib.org/gallery/lines_bars_and_markers/scatter_with_legend.html.

To be discussed:

• Is it a good example?
• Should we have a separate directory/category for "symbols"?
• We should add an alias for -t option (Add common alias transparency (-t) to all plotting functions #614).

Reminders

• Run make format and make check to make sure the code follows the style guide.
• Add tests for new features or tests that would have caught the bug that you're fixing.
• Add new public functions/methods/classes to doc/api/index.rst.
• Write detailed docstrings for all functions/methods.
• If adding new functionality, add an example to docstrings or tutorials.

[seisman]

Is it a good example?

It's a good example for me. It's modified from the matplotlib example, so users can have a direct comparison between pygmt and matplotlib.

Should we have a separate directory/category for "symbols"?

Yes to me. I expect we would have many examples for symbols (different symbol styles, sizes, colors et al.). It's better to group the "symbols" examples together.

[weiji14]

Is it a good example?

It's a good example for me. It's modified from the matplotlib example, so users can have a direct comparison between pygmt and matplotlib.

Fair enough. I think our problem now is that there are too few examples, so the more we have the better! Showcasing the transparency in this example is also good!

Should we have a separate directory/category for "symbols"?

Yes to me. I expect we would have many examples for symbols (different symbol styles, sizes, colors et al.). It's better to group the "symbols" examples together.

Maybe we could place it under the user guide, similar to what is being done for projections at https://www.pygmt.org/v0.2.0/projections/index.html? But that can be discussed in another issue/PR.

[weiji14]

Ah, this is cool, I hadn't thought of using transparent legend boxes before!